The present invention relates to systems for transporting materials, parts, or any other objects between controlled environments without introducing contamination from an intervening uncontrolled environment, and more particularly, to such systems as applied to processes for manufacturing high density electronic integrated circuits.
A serious problem in integrated circuit manufacturing is the presence of particulates and other forms of contamination on and about the wafers on which the circuits are formed. For example, exposure to ambient air gives rise to wafer surface modification, such as by oxidation, nitrification, adsorption of water vapors or other impurities. This surface modification can require additional steps of wafer surface preparation prior to further processing. One of the major sources of particulate contamination is human-generated, including both particles which are released by human bodies and particles which are stirred up by equipment operators moving around inside a semiconductor processing facility. This aspect of the problem has led to the development of various forms of mechanized and automated processing systems, and enclosed-carriers for transporting the wafers between and within such systems. However, the mechanisms themselves are potential generators of particulates. Thus the exclusion of particulates continues to be a limiting factor as device dimensions become smaller and smaller because of the need to avoid the presence of smaller and smaller particles, and because of increased demand for larger integrated circuits.
U.S. Pat. No. 4,995,430 to Bonora et al. discloses a sealable, transportable container that, in one configuration, provides a Standardized Mechanical Interface (SMIF) pod for semiconductor wafers, the pod or carrier including a box, a box door or panel that sealingly closes the bottom of the box, and a wafer cassette that is supported on the panel within the box. A processing station for receiving the carrier comprises a protective canopy having-a loading port, a port door or platform that closes the port being lowerable within the canopy together with the panel and the cassette, the box being retained at the port. The station also includes a manipulator that can move the cassette from the lowered panel for processing of the wafers and then replace the cassette on the panel, the panel being finally raised with the cassette to the box, the reassembled carrier being released from the loading port. The Bonora et al. patent also discloses separate seals between the panel and the box, between the box and the port, and between the platform and the port. A fluid injection/extractor that is mounted to the loading port and fluid-coupled through the box provides means for cleaning the interior of the carrier by alternately evacuating and pressurizing the space within the box. The Bonora et al. patent further discloses that particles that may have been on external surfaces of the panel and the platform are trapped between the panel and the platform by the simultaneous lowering of the panel with the platform.
Many contemporary wafer processing apparatuses are equipped with vacuum load locks. U.S. Pat. No. 5,044,871 to Davis et al. discloses a vacuum wafer carrier that is placed onto a vertically movable stage within an upper chamber of a load lock. When the stage is lowered, a cover of the carrier remains supported by a floor portion of the upper chamber while a cassette of the carrier is lowered into a lower chamber of the load lock. The upper chamber is provided with a load lock cover that is closed after the carrier is placed into the upper chamber. The upper chamber has a vacuum port and a purge port, and a vacuum seal is provided between the floor of the upper chamber and the stage. In one configuration, another vacuum seal is provided between the cover of the carrier and the upper chamber floor. The carrier is represented as maintaining a vacuum during transport and storage, the cover of the carrier being secured by differential pressure.
Notwithstanding the disclosures of Bonora et al. and Davis et al., typical systems for preventing contamination of sensitive articles by particulates and the like at and between process stations remain subject to one or more of the following disadvantages:
1. A relatively large volume must be evacuated prior to processing when the processing is to be performed at less than atmospheric pressure. This is true for the load lock of Davis et al. whether the upper lock chamber or the lower lock chamber is evacuated during a loading cycle. Similarly, Bonora et al. disclose evacuation of the carrier for cleansing the carrier by alternately evacuating and pressurizing the carrier.
2. The lower lock chamber of Davis et al. is subject to contamination from the upper lock chamber and from particulates that may fall onto the stage or that may stick to a bottom surface of the carrier. Similarly, the panel and platform of Bonora et al. are subject to this type of contamination. The contamination is not necessarily trapped between the panel and the platform, but is likely to be dislodged and mobilized when the space between the carrier and the stage or platform is evacuated, the particles moving into an environment that is shared by the wafers during processing.
Thus there is a need for a system that permits transport of wafers by a carrier between controlled environments without these disadvantages.
The present invention meets this need by enabling the contents of a first chamber to be moved into a second chamber without contaminating either chamber with ambient air.
In one aspect of the invention, a system for transporting an object from a first environment into a second environment without contamination by ambient air comprises:
(a) a first housing having a first environment therein, the first housing having a movable wall for supporting the object and for closing the first housing;
(b) a second housing having a second environment therein, the second housing having a movable wall for closing the second housing;
(c) means for locating the first housing relative to the second housing with the movable wall of the first chamber in proximate contact with the movable wall of the second chamber, an interface volume extending between the two movable walls;
(d) a passage for evacuating the interface volume, the passage being isolated from the first and second chambers when the chambers are closed by their respective movable walls;
(e) means for moving the movable wall of the second housing into the second housing; and
(f) means for moving the movable wall of the first housing and the object into the second housing.
Instead of exhausting the interface volume, it can be pressurized through the passage if the chambers are to be pressurized.
Preferably the system includes means preventing the movable wall of the second housing from moving into the second housing until after the interface volume has been evacuated.
The system can further include an interface seal for sealingly connecting the first housing to the second housing, the movable walls of the first and second housing having respective first and second wall seal means for sealingly closing the respective housings, the interface volume being bounded by the interface seal means and the first and second wall seal means when the housings are closed by the respective movable walls.
The interface seal means can include a first elastomeric ring seal on the second housing for contacting an external surface of the second housing. The first wall seal means can include a second elastomeric ring seal on the movable wall of the first housing for contacting an internal surface of the first housing. The third wall seal means can include a third elastomeric ring seal on the movable wall of the second housing for contacting an internal surface of the second housing.
Preferably the interface volume is not more than about 1% of the combined volumes of the first and second chambers for facilitating efficient operation of the system.
The object can be a wafer cassette for holding a plurality of integrated circuit wafers, the first housing can be a wafer carrier enclosure for holding a spaced plurality of wafers, and the second housing can be part of a semiconductor wafer processing machine.
The invention also includes a method of utilizing the system, wherein the contents of the first chamber are moved into the second chamber by the steps of:
(a) placing the movable wall of the first chamber in proximate contact with the movable wall of the second chamber, with the interface volume extending between the two movable walls;
(b) evacuating the interface volume or adding gas through the interface volume through the passage;
(c) moving the movable wall of the second chamber into the second chamber; and
(d) moving the movable wall of the first chamber and the contents of the first chamber into the second chamber.
Generally the walls are moved simultaneously for preventing any contamination that might remain between the movable walls from contaminating the second chamber, and to prevent generation of contamination. The two movable walls can be kept in contact during the steps (c) and (d) of moving the walls, i.e., the walls are moved together. The movable wall of the second chamber can be horizontally disposed, wherein the steps (c) and (d) of moving the walls comprises lowering the walls.
Preferably, when the object is to be subjected to reduced pressure in the second chamber, the method includes the further steps of maintaining the chambers in respective evacuated conditions. Thus the invention eliminates the need for evacuating large volumes of air from the chambers while safely purging the facing surfaces and any volume between the movable walls.
By maintaining the two chambers continuously in an evacuated condition, there is no requirement for evacuating a large volume of air in preparation for a process step to be done under vacuum. Further, the interface volume can be quickly evacuated to a destination separate from either chamber, thereby avoiding contamination of the chambers, particularly the second chamber, by contaminants that might otherwise be mobilized from between the movable walls during evacuation.
When the first chamber is initially at a superatmospheric or subatmospheric pressure, the pressure in the second chamber is controlled to approximately match the initial first chamber pressure. It is preferred that the method include the additional step of preventing movement of the wall of the second chamber until the interface volume has been evacuated or pressurized sufficiently to approximately equalize the pressure of the interface volume with that of the second chamber. The step of preventing the movable walls from moving can be continued until the interface volume reaches approximately the initial first chamber pressure.